In the prior art, there are book binding systems provided with a sheet feeder which feeds sheets one by one from a sheet stack outputted from a printer or a copier and formed of a plurality of individual sheet bundles each of which consists of collated sheets and corresponds to one book block, and a book binding unit which receives the sheets from the sheet feeder and binds each of the sheet bundles into a booklet or a book. Also there are book binding system provided with a collator which produces a sheet stack formed of a plurality of individual sheet bundles each of which consists of collated sheets and corresponds to one book block, a sheet feeder which feeds sheets one by one from the sheet stack, and a book binding unit which receives the sheets from the sheet feeder and binds each of the sheet bundles into a booklet or a book.
According to such book binding systems, it is necessary to detect boundaries of the sheet bundles of the sheet stack and conduct the sheet feed from the sheet feeder to the book binding unit for each sheet bundle.
Therefore, there has been a sheet feeder which automatically detects the boundaries of the sheet bundles of the sheet stack and stops the sheet feed temporarily whenever the sheet feed of one sheet bundle is completed (also see, for example, JP 1999-322201 A).
This sheet feeder has, as shown in FIG. 5, a shelf 30 movable in a vertical direction and supporting a sheet stack 31 thereon, a first conveyance unit A arranged above and opposite to the uppermost sheet of the sheet stack 31 on the shelf 30 so as to convey the uppermost sheet forward by engaging with the uppermost sheet, and a second conveyance unit B arranged at the downstream of the first conveyance unit A so as to receive the sheet from the first conveyance unit A and discharge the sheet to the book binding apparatus.
The first conveyance unit A is composed of a feed roller 32, a drive sprocket 34 fixed to a drive shaft of a motor 33, an auxiliary roller 35, and a drive belt 36 extending among the feed roller 32, the drive sprocket 34 and the auxiliary roller 35.
While the shelf 30 is gradually raised, the periphery surface of the feed roller 32 is rotated by the motor 33 and feeds the uppermost sheet of the sheet stack 31 forward by engaging with the uppermost sheet.
The second conveyance unit B is composed of a conveying belt 41 extending between two parallel idle rollers 37, 38, a drive sprocket 40 fixed to a drive shaft of a motor 39, a drive belt 42 extending between the drive sprocket 40 and the idle roller 38, and feed rollers 43 arranged above the conveying belt 41 and spaced from one another in a sheet feed direction.
The idle roller 38 is rotated by the motor 39 through the drive belt 42, and thereby the conveying belt 41 is circulated so that the sheet fed from the first conveyance unit A is conveyed by the conveying belt 41 and discharged to a sheet accumulating unit 46 of the book binding machine.
The vertical movement of the shelf 30 and the operation of each of the first and second sheet conveyance units A, B are controlled by a control unit (not shown).
At the beginning of the conveying belt 41, that is, the second conveyance unit B, a sensor 44 is arranged to count the sheets delivered from the first conveyance unit A to the second conveyance unit B. Furthermore, an image sensor 45 is arranged at the downstream of the sensor 44 to take images of printed areas of the sheets.
Although not shown the drawing, the sheet feeder has a memory and images of printed areas of the first sheets of the sheet bundles are pre-stored as reference data.
Thus, after the sheet feeder starts its operation, the sheets fed to the conveying belt 41 are counted by the sensor 44. When all of the sheets of one sheet bundle are counted and the next sheet (corresponding to the first sheet of the next sheet bundle) is counted, the control unit compares between the image data of the printed area of the such sheet outputted from the image sensor 45 and the reference data. In the case of an agreement between the image data and the reference data, the control unit stops the first conveyance unit A temporarily so as to stop the feed of the such sheet to the second conveyance unit B. During this momentary pause, the previous sheet bundle is bound into a booklet or a book by the book binding apparatus.
However, in this conventional sheet feeder, the image sensor 45 takes the images of the sheets while the sheets are conveyed by the conveying belt 41 and therefore, in order to obtain a high-definition and sharp image, it is necessary to convey the sheets at sufficient low speed. Therefore, there is a limit on speeding up of the sheet feed.
In addition, according to this conventional sheet feeder, the image sensor 45 continuously takes the images of the sheets while the sheets are conveyed by the conveying belt 41, and the comparison between the image data outputted from the image sensor 45 and the reference data should be made by the control unit at the same time as the image sensor 45 takes the image of the sheet within a range of the sheet corresponding to the printed area of the first sheet of the sheet bundle. Therefore, the control unit receives the image from the image sensor 45 at a timing predetermined based on the velocity of the sheet and the positional information of the image sensor 45.
However, the velocity of the sheet fluctuates easily depending on the variety and thickness and so on of the sheet and thereby the timing of the image capture by the control unit from the image sensor is deviated, which possibly causes a detection error that the comparison between the image data containing no image to be compared and the reference data is made by the control unit. Such detection error may be avoided by broadening a sensing area of the image sensor 45. But if the sensing area of the image sensor 45 is broaden, the image data outputted from the image sensor 45 contains a great deal of unnecessary image as well as the image to be compared and an image processing of extracting the image data to be compared from this outputted data needs a lot of time and thereby the speeding up of the sheet feed is restricted.